1. Field of the Invention
This invention relates to a component for coupling optical energy out of an optical fiber and, particularly to an optical fiber tap that greatly reduces overall size and volume of the tap compared to existing fused-fiber taps while maintaining high coupling efficiency to photodiodes for power monitoring applications in fiber optic systems.
2. Description of the Prior Art
The growth of optical fiber amplifiers and wavelength-division multiplexing (WDM) techniques in fiber optic systems has led to an increase in a number of active fiber optic components deployed in commercial telecommunications networks. With this development has come a need for monitoring devices that can provide information on the performance of these active components in order to maintain system performance levels and quickly address system faults when they occur. Such monitoring devices need to be highly reliable, low in cost, and small in size, the last property being of increasing importance as fiber optic equipment manufacturers compete to put more functionality into ever smaller spaces.
One of the most important monitoring functions in fiber optic systems is monitoring of optical power levels at various points in a fiber optic network. By monitoring optical power one obtains a good, though incomplete, indicator of system performance. For example, optical power is typically monitored both at an input and an output of optical fiber amplifiers to provide information on gain and saturation of the amplifier. In many cases, optical power from a laser that pumps the amplifier is also monitored. Another example is monitoring of optical power entering a receiver, such monitoring being necessary to insure that the receiver does not become saturated and thus performance degraded.
Known means for monitoring the optical power being carried by an optical fiber require the use of a tap to remove a small fraction of the optical power traveling in the fiber. The tapped-out light is typically sent to a photodiode that converts its optical signal to an electrical signal which is then processed electronically. Provided the ratio of optical power removed from the fiber to optical power remaining in the fiber is a fixed number, the electrical signal generated by the photodiode can serve as a measure of the optical power flowing in the fiber. Ideally, most of the optical power entering the tap passes through to the output and is unaffected by its presence.
Among known fiber optic taps, by far the most common is the fused fiber optic coupler formed by fusing two optical fibers together. In such a device, cores of the two fibers after fusing are sufficiently close in proximity that light traveling in one fiber is partially transferred to the other fiber, the former fiber being referred to as the xe2x80x9cthrough legxe2x80x9d and the latter being referred to as the xe2x80x9ctap-legxe2x80x9d. In monitoring applications the light in the tap-leg, which is typically of less power than the light in the through-leg, is sent to a photodiode to generate an electrical signal. Fused fiber couplers are well known in the art and have been made to exhibit a variety of properties. See, for example, U.S. Pat. No. 4,426,215 (issued to K. A. Murphy on Jan. 17, 1984); U.S. Pat. No. 5,011,251 (issued to W. J. Miller et al on Apr. 30, 1991); and U.S. Pat. No. 5,251,277 (issued to D. R. Young on Oct. 5, 1993).
Fused fiber couplers suffer from a number of disadvantages when used in power monitoring applications. First among these is a necessity of terminating four fiber ends (two ends for each fiber). A power monitor is a three port device consisting of an optical input, an optical output and an electrical output. When constructing power monitors using fused fiber couplers, it is necessary to terminate the tap-leg to the photodiode and also terminate the unused input port. In manufacturing fiber optic components, termination of fiber ends is a significant contributor to labor costs.
A second disadvantage of fused fiber couplers is their physical size. Although their packaging volume can be small, they tend to be elongated in one dimension owing to the need to fuse the two fibers together over a sufficient length to obtain the desired coupling without inducing excessive optical loss. This puts a practical lower limit on the size of fused fiber couplers and makes their integration into optoelectronic modules rather difficult. In addition, termination of the tap-leg to a photodiode for monitoring optical power requires a further increase in a longest package dimension. A still further limitation on the physical size of fused fiber couplers arises from a need to add a protective housing and substrate for the fibers after they are fused together owing to the fragile condition of the fused fibers.
Another known means for making an optical fiber tap is to induce a microbend in the fiber. The microbend causes a fraction of optical power to scatter out of the side of the fiber. In power monitoring applications the scattered light is directed on to a photodiode by means of mirrors or lenses. Examples of optical fiber taps using microbending are given in U.S. Pat. No. 5,037,170 (issued to W. D. Uken et al on Aug. 6, 1991); U.S. Pat. No. 5,039,188 (issued to G. F. Williams on Aug. 13, 1991); and U.S. Pat. No. 5,708,265 (issued to C. D. Poole on Jan. 13, 1998). The primary disadvantages of these optical taps are the need for additional optical components to collect the light that emerges from the side of the fiber onto a photodiode and a nonlinear trajectory of the fiber caused by the bending. Both of these features make integrating such devices into relatively small packages with photodiodes difficult and thus costly.
The present invention provides an optical tap that is highly reliable, small in size, and that can be integrated into miniature opto-electronic packages.
The invention accomplishes this by overcladding an optical fiber using a glass tube with a reflecting surface at one end. Optical energy is tapped out of the optical fiber by exciting cladding modes in the fiber just upstream of the glass tube. In the preferred embodiment the cladding modes are excited by inducing a taper in the fiber. By fusing the glass tube to the fiber, the cladding modes are able to enter the glass matrix of the tube and are reflected out by the reflecting surface at the downstream end of the glass tube. The reflecting surface is formed by polishing the that end of the glass tube at an approximate angle of 45 degrees prior to assembly. Through total internal reflection this surface reflects the cladding modes out at an approximate angle of 90 degrees to the fiber axis, thus making collection onto a photodiode easy and efficient.
In particular and in accordance with my inventive teachings, the present invention provides a means for tapping optical power out of a fiber, wherein the preferred embodiment comprises a silica tube fused to the optical fiber and having one end polished at an angle, a short segment of the optical fiber tapered down to induce cladding modes and an encapsulating tube of silica to enclose the assembly hermetically. According to my inventive technique, the silica tube is doped with GeO2 so as to lower its softening point temperature and prevent excess loss when fusing to the optical fiber, while at the same time maintaining a refractive index in the silica tube equal to or higher than the cladding of the optical fiber.
According to my inventive technique, a device for monitoring optical power in an optical fiber is made by integrating the optical tap assembly into a photodiode package so that cladding mode light that is reflected out of the optical fiber falls on the photodiode, thus generating an electrical signal that represents the optical power flowing in the optical fiber.
An additional embodiment that makes use of my inventive teachings comprises a silica capillary tube that is preferentially doped with GeO2 in a narrow annular region. This doped region, when the capillary tube is collapsed and fused onto the optical fiber, acts as a waveguide to collect the cladding light emerging from the fiber and deliver it to the photodiode at increased efficiency and with reduced wavelength dependence.